Well, most important if you're the kind of person who's still playing TF2. Because if you are, you probably enjoy hats.

The robot wars will include a game mode called Mann Up. Completing these maps (which can be linked together to form "tours of duty") gets you rare loot in the forms of attire and accessories for your characters. But to get onto those maps you have to pay.

Think of it as an amusement park ride. You pay $0.99 for a ticket (this isn't part of the metaphor, it's actually what you pay and what you get), and that ticket is used to get on an official Valve Mann Up server. Once there, you're in line to get the rare loot.

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Note that this is purely for cosmetic purposes; the Mann vs Machine game modes are free to play on any non-Mann Up server. And those who do pay are only getting superficial items, they won't be getting better perks or more powerful weapons

Still...this is a game whose entire economy is built around hats. Now that there are better and newer hats, making money shouldn't be a problem for Valve.